how and when do you prune cornus

You can cut it back very hard in the late winter/early spring. This will ensure that it sends up new coloured stems ready to enjoy next winter.

yes, as Spritz says, you should cut it back to ALMOST ground level in late winter/early spring every year.

Yes you can cut back hard anytime next few weeks, also cot one in five stems to ground level to, i do this with mine and this winter it has been lovely.